The present invention relates to apparatus for treating foils or like sheets of thermoplastic material, and more particularly to improvements in apparatus for severing and simultaneously heat sealing (thermowelding) overlapping portions of synthetic thermoplastic sheets. Still more particularly, the invention relates to improvements in apparatus of the type wherein a hold down device is provided to bias one of the sheets against the other sheet and to thereby bias the other sheet against a preferably stationary anvil or an analogous support in the course of and preferably for a selected interval following a combined severing and thermosealing operation which is performed by a heated tool.
Apparatus of the above outlined character are often utilized to confine goods in envelopes consisting of two synthetic plastic sheets whose marginal portions are integral with or are welded to each other. For example, the confined goods can constitute discrete steno pads, stacks of two or three superimposed steno pads or other stationary products. As a rule, the tool includes a holder for a continuously heated knife blade whose cutting edge is caused to bear against the exposed side of one of the overlapping synthetic thermoplastic sheets and to thereby urged the exposed side of the other sheet against an anvil or an analogous support. In lieu of a knife, the tool can employ an impulse-heated wire which is used to sever the sheets as well as to bond the sheets to each other at both sides of the cutting plane. The hold down device is normally made of sheet steel and the apparatus may but need not be provided with means for cooling the hold down device.
The purpose of the hold down device is to ensure that the sheets lie flat against the support and do not exhibit any folds or wrinkles in the region of the cutting plane. Moreover, the hold down device is normally called upon to bias the severed parts of the overlapping sheets against the support for a certain interval of time following the combined severing and heat sealing operation so as to allow the freshly bonded marginal portions of the sheets to dissipate heat and to thus form reliable welded seams, one at each side of the severing plane.
If the hold down device is made of sheet steel and does not embody or does not cooperate with a cooling unit, the material of the hold down device is rapidly heated to a temperature at which the material of the sheets cannot set in the regions of contact with the hold down device. On the contrary, the latter plasticizes the contacted portions of the adjacent sheet which affects the quality of the severing action and necessitates relatively long intervals for cooling of the freshly formed welded seams. If the seams are to be formed in the gaps between relatively tall stacks of steno pads or the like, the portions of sheets in such gaps are likely to adhere to the corresponding sections of the hold down device and to melt. This entails rapid contamination of the hold down device as well as of the tool so that the operation must be interrupted at frequent intervals in order to clean the hold down device and the tool. Moreover, the quality of the severing and thermosealing operation deteriorates proportionally with the increasing accumulation of plastic material on the hold down device and/or on the tool.
For the foregoing reasons, hold down devices which are not equipped or combined with cooling means cannot be used with permanently heated blade-like combined severing and heat sealing tools. They are merely useful in conjunction with tools which employ impulse-heated wires, i.e., wires which are heated at a certain frequency, always when they come in contact with the thermoplastic material. Moreover, the frequency at which the wire is heated in pulsating fashion cannot be too high because this would entail excessive heating of the hold down device. Therefore, such apparatus cannot be utilized in high-speed production lines which turn out stationery products or other goods in large numbers per unit of time and wherein the combined severing and heat sealing tool must be continuously heated or must be heated at a high or very high frequency.
Attempts to solve the aforediscussed problems include the provision of a variety of cooling means for the hold down device. The cooling means can operate with circulating water, a suitable emulsion or a gaseous fluid. This, in turn, contributes significantly to the bulk and cost of the apparatus. Moreover, the energy requirements of such cooling systems are substantial because they must be equipped with pumps or other suitable means for circulating large quantities of a gaseous or liquid coolant per unit of time. As a rule, the width or thickness of a cooled hold down device substantially exceeds the width or thickness of an uncooled device. Consequently, neighboring goods which are confined intermediate two synthetic plastic sheets and must be sealed from the surrounding atmosphere by making welded seams therebetween cannot be placed close to each other because the gaps between neighboring goods must be sufficiently wide to provide room for the introduction of bulky hold down devices. Therefore, pouches or like envelopes which are obtained upon completion of the heat sealing operation are larger than necessary which presents problems during packing of such goods in containers and entails unnecessary loses in synthetic plastic material. As a rule, such pouches are thereupon caused to shrink in order to closely follow the outlines of the confined goods. This is not possible if the dimensions of the pouches are excessive, i.e., the pouches are likely to wrinkle which detracts from the appearance and sales appeal of the confined goods. Unnecessary losses in the material of synthetic plastic foils are very pronounced if the apparatus is to confine large numbers of goods in discrete synthetic plastic envelopes which are larger than absolutely necessary and desirable for the purposes of obtaining envelopes which closely hug the outlines of the confined goods.
Another drawback of cooled hold down devices is that they withdraw a certain amount of heat energy from the combined severing and heat sealing tool, i.e., the energy requirements of the means for heating the tool are higher than necessary and the energy requirements of the means for withdrawing heat from the cooling medium (if the cooling medium is recirculated) are also higher than necessary.
A further drawback of heretofore known hold down devices (which often comprise pairs of rigid plates made of steel or another metallic material and flanking the blade or wire of the combined severing and heat sealing tool) is that the plates merely bear upon the exposed side of the adjacent sheet and press it against the other sheet which, in turn, is pressed against the support. Thus, when the sheets are severed in a cutting plane which is located between the two plates and the plates continue to bias the thus obtained parts of the sheets against the support, the freshly formed welded seams are likely to adhere to the support. If the mass of the confined goods is not substantial, such goods cannot readily separate the freshly welded seams from the support so that the products which adhere to the support must be detached and removed in a time-consuming operation which can lead to a complete shutdown of an entire production line.
Commonly owned copending patent application Ser. No. 678,236 filed Dec. 5, 1984 by Bernd Ramcke discloses an apparatus wherein the combined severing and thermosealing tool is pivotable to and from an operative position in which it severs and bonds travelling sheets of thermoplastic material. The severing station is cooled by a jet of compressed air issuing from a nozzle.